Abstract

In this work a novel type of impedance controllers
for flexible joint robots is proposed. As a target impedance a
desired stiffness and damping are considered without inertia
shaping. For this problem two controllers of different complexity
are proposed. Both have a cascaded structure with an inner
torque feedback loop and an outer impedance controller. For
the torque feedback, a physical interpretation as a scaling of the
motor inertia is given, which allows to incorporate the torque
feedback into a passivity based analysis. The outer impedance
control law is then designed differently for the two controllers.
In the first approach the stiffness and damping terms and the
gravity compensation term are designed separately. This outer
control loop uses only the motor position and velocity, but no noncollocated
feedback of the joint torques or link side positions. In
combination with the physical interpretation of torque feedback,
this allows us to give a proof of the asymptotic stability of the
closed-loop system based on the passivity properties of the system.
The second control law is a refinement of this approach, in which
the gravity compensation and the stiffness implementation are
designed in a combined way. Thereby, a desired static stiffness
relationship is obtained exactly. Additionally, some extensions of
the controller to visco-elastic joints and to Cartesian impedance
control are given. Finally, some experiments with the DLR
lightweight robots verify the developed controllers and show the
efficiency of the proposed control approach.